Storm-induced turbidity currents on a sediment-starved shelf: Insight from direct monitoring and repeat seabed mapping of upslope migrating bedforms |
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| Authors: | Alexandre Normandeau Daniel Bourgault Urs Neumeier Patrick Lajeunesse Guillaume St-Onge Louis Gostiaux Cédric Chavanne |
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| Institution: | 1. Geological Survey of Canada – Atlantic, 1 Challenger Drive, Dartmouth, Nova Scotia, B2Y 4A2 Canada;2. Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, Rimouski, Québec, G5L 3A1 Canada;3. Département de Géographie, Centre d’études Nordiques, Université Laval, 2405 Rue de la Terrasse, Québec, Québec, G1V 0A6 Canada;4. Univ Lyon, École Centrale de Lyon, INSA Lyon, CNRS, Laboratoire de Mécanique des Fluides et d'Acoustique, UMR 5509, Université Claude Bernard Lyon I, 36 Avenue Guy de Collongue, F-69134 Ecully, France |
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| Abstract: | The monitoring of turbidity currents enables accurate internal structure and timing of these flows to be understood. Without monitoring, triggers of turbidity currents often remain hypothetical and are inferred from sedimentary structures of deposits and their age. In this study, the bottom currents within 20 m of the seabed in one of the Pointe-des-Monts (Gulf of St. Lawrence, eastern Canada) submarine canyons were monitored for two consecutive years using Acoustic Doppler Current Profilers. In addition, multibeam bathymetric surveys were carried out during deployment of the Acoustic Doppler Current Profilers and recovery operations. These new surveys, along with previous multibeam surveys carried out over the last decade, revealed that crescentic bedforms have migrated upslope by about 20 to 40 m since 2007, despite the limited supply of sediment on the shelf or river inflow in the region. During the winter of 2017, two turbidity currents with velocities reaching 0·5 m sec?1 and 2·0 m sec?1, respectively, were recorded and were responsible for the rapid (<1 min) upstream migration of crescentic bedforms measured between the autumn surveys of 2016 and 2017. The 200 kg (in water) mooring was also displaced 10 m down-canyon, up the stoss side of a bedform, suggesting that a dense basal layer could be driving the flow during the first minute of the event. Two other weaker turbidity currents with speeds <0·5 m sec?1 occurred, but did not lead to any significant change on the seabed. These four turbidity currents coincided with strong and sustained wind speed >60 km h?1 and higher than normal wave heights. Repeat seabed mapping suggests that the turbidity currents cannot be attributed to a canyon-wall slope failure. Rather, sustained windstorms triggered turbidity currents either by remobilizing limited volumes of sediment on the shelf or by resuspending sediment in the canyon head. Turbidity currents can thus be triggered when the sediment volume available is limited, likely by eroding and incorporating canyon thalweg sediment in the flow, thereby igniting the flow. This process appears to be particularly important for the generation of turbidity currents capable of eroding the lee side of upslope migrating bedforms in sediment-starved environments and might have wider implications for the activity of submarine canyons worldwide. In addition, this study suggests that a large external trigger (in this case storms) is required to initiate turbidity currents in sediment-starved environments, which contrasts with supply-dominated environments where turbidity currents are sometimes recorded without a clear triggering mechanism. |
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| Keywords: | Acoustic doppler current profiler bedforms cyclic steps monitoring seabed mapping St Lawrence Estuary storms turbidity currents |
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